Listen-Before-Talk (LBT) and Carrier Sense Multiple Access (CSMA)
In wireless networks operating in unlicensed spectrum, multiple nodes may access the wireless medium simultaneously without the arbitration of a central entity. In such situations, a distributed Medium Access Control (MAC) mechanism can help the wireless nodes to coordinate the access and provide an arbitration mechanism such that each node can transmit its packet with low probability of packet collisions. Examples of such mechanisms are the CSMA and LBT protocols employed by wireless technologies operating in unlicensed spectrum.
For Long Term Evolution (LTE) systems deployed in unlicensed spectrum, using the LBT protocol enables coexistence with, for example, IEEE wireless networks, i.e., WiFi networks using CSMA based MAC. LTE systems employing LBT do not degrade the performance of the IEEE system more than if an additional IEEE network was deployed in the same frequency band. Thus, LBT and CSMA are important mechanisms in environments of mixed wireless technologies.
A well-known problem associated with CSMA and LBT protocols is the hidden node problem illustrated in FIG. 1. The hidden node problem may lead to packet collisions at the receiver and thereby throughput degradations and instability of the system. In the illustrated example, a first User Equipment device (UE-A) has an ongoing transmission in the unlicensed band. A second UE (UE-B) is far enough from UE-A that UE-B does not sense the transmission of UE-A when performing CSMA/LBT and, as such, UE-B also begins transmitting in the unlicensed band. As a result, there is packet collision at the intended receiver, which results in a degradation in throughput and instability of the system.
Another problem in CSMA and LBT systems is the so called exposed node problem illustrated in FIG. 2. An exposed node senses the channel busy when it is close to a wireless transmitter and refrains from transmission even when its intended receiver node is interference free. In the illustrated example, there is an ongoing transmission from UE-A to base station A (BS-A) in the unlicensed band. UE-B desires to transmit in the unlicensed band to BS-B. However, during CSMA/LBT, UE-B detects the transmission from UE-A (i.e., senses energy in the channel) and therefore determines that the channel is busy. However, this is an undesirable result because the intended receiver for UE-B is different than the intended receiver for the transmission from UE-A. In other words, UE-B could have transmitted to BS-B in the unlicensed band without causing interference to the transmission from UE-A to BS-A.
A consequence of the hidden and exposed node problems is a throughput degradation due to retransmissions and delayed transmissions. Also, the overall load in terms of transmitted packets and served UEs that the system can handle without diverging packet delivery times can be severely degraded. Recognizing the importance of solving the hidden and exposed node problems, existing technologies provide wireless protocols such as, e.g., those based on Ready to Send (RTS) and Clear to Send (CTS) signaling that help mitigate these problems at the expense of extra complexity and protocol overhead.
License Assisted Access (LAA) in Unlicensed Spectrum
Currently, unlicensed frequency bands provide the possibility to enhance the capacity of cellular networks operating solely in licensed bands. To this end, existing and currently standardized technologies such as Third Generation Partnership Project (3GPP) LTE-Unlicensed (LTE-U) and LAA provide mechanisms by means of which LTE UEs can access the unlicensed spectrum bands without implementing IEEE technologies. Such UEs must adhere to regulations that facilitate the coexistence of 3GPP and IEEE technologies in the same frequency bands. UEs implementing LAA and LTE-U protocols must comply with various regulatory requirements concerning the employed duty cycle, transmit power levels, and medium access mechanisms, such as carrier sensing and Clear Channel Assessment (CCA) so that a fair sharing of the wireless medium by multiple technologies is achieved.
The primary objective of LTE-U and LAA technologies is increasing the capacity in scenarios in which LTE networks operating in licensed bands are deployed. Public indoor deployments, outdoor hotspots, or corporate environments, for example, can benefit from such capacity enhancements.